Recently, thanks to the development of telecommunication technology, mobile terminals, etc. are widely used. Such mobile terminals demand a high efficiency of transmitting RF signals. For the transmission of RF signals, the power amplifier which consumes the largest portion of DC power may be handled as the critical component to determine overall RF transmitter efficiency. Accordingly, power amplifiers with a high efficiency are necessary to improve overall efficiency
In general, class-E power amplifiers are more widely used due to benefits such as high efficiency and simplicity of realization. The class-E power amplifier was introduced by Sokal in 1975 and experimented by Raab. In more detail, as a kind of switching amplifier, it may achieve ideal 100% drain (or collector) efficiency by shaping a voltage waveform and a current waveform. So far, circuit designs for new devices, such as As HBTs, GaN HEMTs, and InP DHBTs (Double HBTs), used to operate on higher frequency ranges and microwave with higher RF performances have been actively developed. Recently, researches have been conducted about watt-level output power amplifiers using CMOS technology on microwave frequency ranges to reduce costs.
The CMOS power amplifier may reduce design costs but bring several problems. One of the problems is a low breakdown voltage. As a process scale becomes smaller, the breakdown voltage is also reduced. As the result, power supply voltage is limited to avoid breakdowns. To deliver a wide range of output power under the low power supply voltage condition, it is inevitable to reduce load impedance. But this causes efficiency degradation and a narrowband load matching network.
Additionally, the CMOS power amplifier needs to reduce on-resistance for high efficiency and to drive sufficient currents for watt-level output power. Therefore, it requires a wide gate width. The maximum operating frequency fMAX of the class-E power amplifier is proportional to peak drain current IMAX. Accordingly, the gate width cannot but reach some millimeters. Even though the gate width is widened, this causes parasitic elements such as parasitic capacitance. The parasitic capacitance increases shunt drain capacitance CP, which limits the normal operation condition because the maximum operating frequency fMAX of the class-E power amplifier is determined by an IMAX/CP ratio. When the shunt drain capacitance CP deviates from the optimal value determined by the condition of the class-E power amplifier operation due to the parasitic capacitance, etc., the efficiency degradation occurs.